Ink jet printing involves ejecting ink droplets from orifices in a print head onto a receiving surface to form an image. The image is made up of a grid-like pattern of potential drop locations, commonly referred to as pixels. Ink-jet printing systems commonly utilize either a direct printing or offset printing architecture. In a typical direct printing system, ink is ejected from jets in the print head directly onto the final receiving web. In an offset printing system, the image is formed on an intermediate transfer surface and subsequently transferred to the final receiving web. The intermediate transfer surface may take the form of a liquid layer that is applied to a support surface, such as a drum. The print head jets the ink onto the intermediate transfer surface to form an ink image thereon. Once the ink image has been fully deposited, the final receiving web is then brought into contact with the intermediate transfer surface and the ink image is transferred to the final receiving web.
U.S. Pat. No. 5,389,958, assigned to the assignee of the present application, is an example of an indirect or offset printing architecture that utilizes phase change ink. The ink is applied to an intermediate transfer surface in molten form, having been melted from its solid form. The ink image solidifies on the liquid intermediate transfer surface by cooling to a malleable solid intermediate state as the drum continues to rotate. When the imaging has been completed, a transfer roller is moved into contact with the drum to form a pressurized transfer nip between the roller and the curved surface of the intermediate transfer surface/drum. A final receiving web, such as a sheet of media, is then fed into the transfer nip and the ink image is transferred to the final receiving web.
One form of direct-to-sheet, continuous-web, phase-change solid ink printer is disclosed in pending application Ser. No. 11/773,549, filed on Jul. 5, 2007, and published as U.S. No. 2009/0009573, assigned to the assignee of the present application, which disclosure is incorporated herein by reference One embodiment of a direct-to-sheet printer is depicted in FIG. 1. In this printer, a substantially continuous web W or “substrate” (such as paper, plastic, or other printable material) is conveyed through a path by a series of conveying components, such as rollers. The path includes a pre-heater 12 that brings the web to an initial predetermined temperature. The web W is conveyed by the components through a printing station 10 that includes a series of printheads 14 configured to place a phase-change ink of one primary color directly onto the moving web.
The ink directed onto web is a solid “phase-change ink,” by which is meant that the ink is substantially solid at room temperature and substantially liquid when initially jetted onto the web W. Common phase-change or solid inks are typically heated to about 100° C. to 140° C., and thus in liquid phase, upon being jetted onto the web. Generally speaking, the liquid ink cools down quickly upon hitting the web W.
Associated with each printhead is a backing member 16, typically in the form of a bar or roller, which is arranged substantially opposite the printhead 14 on the other side of and supporting the web W. Each backing member 16 can be heated and controlled, in combination with the pre-heater, to cause the adjacent portion of the web to reach a predetermined “ink-receiving” temperature, for instance about 40° C. to about 70° C. The phase-change or molten solid ink is jetted at a temperature typically significantly higher than the receiving web's temperature, often in the range of 100-140° C., so in some cases the web temperature is further controlled by utilizing air blowers or fans behind the web in the printing station.
Following the printing station the web is conveyed along the path by a series of tension rollers, followed by one or more “mid-heaters” 18. The mid-heaters bring the ink placed on the web to a temperature suitable for desired properties when the ink on the web is sent through a subsequent “spreader” component 20. The spreader component 20 applies a predetermined pressure, and in some implementations heat, to the web to take what are essentially isolated droplets of ink on the web and smear them out to make a continuous layer by pressure. The spreader typically includes opposing rollers, such as an image-side roller 22 and a pressure roller 24. In one practical embodiment disclosed in the aforementioned application Ser. No. 11/773,549, the nip pressure between the two rollers is set in a range of about 500 to about 2000 psi lbs/side. Lower nip pressure gives less line spread while higher nip pressure may reduce roller life.
The spreader may also include a cleaning/oiling station 26 associated with image-side roller that is suitable for cleaning and/or applying a layer of some lubricant or other material to the roller surface. Such a station 26 coats the surface of the spreader roller with a lubricant such as an amino silicone oil having viscosity of about 10-200 centipoises. Following the spreader, some printers include a “glosser”, whose function is to change the gloss of the image or impress a desired surface texture. In certain machines that permit duplex printing, a turn roller 28 may be provided between the mid-heater and the spreader, as well as at the beginning of the printing path. In a certain printer, twenty-four backing rollers 16 and two turn rollers 28 are provided.
In a typical direct printing machine, the pressure rollers 24 are formed of a relatively soft material with a durometer anywhere from about 50 D to about 65 D, with elastic moduli from about 65 MPa to about 115 MPa. In contrast, the opposing image side rollers 22 that contact the inked side of the web are typically formed of a relatively hard material, such as a metal. In certain embodiments the rollers 22 are formed of anodized aluminum. Similarly, the backing rollers 16 and the turn rollers 28 are formed of the same material, namely anodized aluminum.
Each of the anodized aluminum rollers is in contact with spread and un-spread sold ink images depending upon their location in the printing path and on whether the process is simplex or duplex. It is desirable in any printing machine to minimize the amount of ink that is offset from the substrate or web onto the rollers. In printer architectures such as described above, ink offset onto an aluminum roller will occur when the adhesive force between the ink image and the roller is stronger than the cohesive force within the ink image itself. One approach to minimizing ink offset is to maintain the rollers at a relative low temperature, in the neighborhood of 30° C. Since the temperature of the ink itself is much higher than this desired temperature, cooling fans are necessary to reduce the web and ink temperature at the printing stations. The web and ink temperature must then be increased to around 60° C. at the spreader for optimal spreading of the ink onto the web. The result is a process with a narrow range of operation that can be energy inefficient.
Consequently, there is a need for a roller construction that reduces the risk of ink offset onto the roller under conditions that optimize the printing process and energy efficiency of the process. There is also a need for low adhesion coatings that show little affinity or have low adhesion towards the solid ink image.